WASHINGTON — The Justice Department lent its support on Thursday to students who are suing Harvard University over affirmative action policies that they claim discriminate against Asian-American applicants, in a case that could have far-reaching consequences in college admissions.

“Harvard has failed to carry its demanding burden to show that its use of race does not inflict unlawful racial discrimination on Asian-Americans,” the Justice Department said in its filing.

The filing said that Harvard “uses a vague ‘personal rating’ that harms Asian-American applicants’ chances for admission and may be infected with racial bias; engages in unlawful racial balancing; and has never seriously considered race-neutral alternatives in its more than 45 years of using race to make admissions decisions.”

The Justice Department has increasingly used such statements of interest to intervene in civil rights cases. Before 2006, such statements appeared only seven times in civil rights-oriented disputes, according to a recent paper by law school student Victor Zapana. From 2006 to 2011, they were drafted in at least 242, almost all by the Obama administration on issues such as videotaping police brutality and ensuring that blind people and their service dogs have access to Uber.

But the Trump administration is turning the same tool against affirmative action in college admissions, a major — and highly contentious — legacy of the civil rights era, and one that white conservatives have opposed for decades. In the past few years, the anti-affirmative action cause has been joined by Asian-Americans who argue that they are being held to a higher standard, losing out on coveted slots at places like Harvard as African-Americans, Latinos and other groups get a boost.

A handful of states already ban public universities from relying on affirmative action, pushing several toward a model that takes socioeconomic factors into account instead of race. Public universities in California and Washington have tried to engineer class-based diversity in their student bodies, believing that giving a lift to lower-income students will end up bringing in more minority students as well.

Now, universities that factor race into admissions have found a powerful new opponent in the Trump administration, which argued in its filing on Thursday that the court should deny Harvard’s request to dismiss the case before trial.

The government said that Supreme Court rulings require that universities considering race in admissions meet several standards. They must define their diversity-related goals and show that they cannot meet those goals without using race as a factor in admissions decisions.

The department argued that Harvard does not adequately explain how race factors into its admissions decisions, leaving open the possibility that the university is going beyond what the law allows.

“Harvard has failed to show that it does not unlawfully discriminate against Asian-Americans,” the Justice Department said in a statement Thursday.

Harvard said it was “deeply disappointed” but not surprised “given the highly irregular investigation the D.O.J. has engaged in thus far.”

“Harvard does not discriminate against applicants from any group, and will continue to vigorously defend the legal right of every college and university to consider race as one factor among many in college admissions, which the Supreme Court has consistently upheld for more than 40 years,” the university said in a statement.

A broad coalition of Harvard supporters filed briefs in support of the school Thursday condemning the lawsuit and saying that it would effectively threaten diversity at all American colleges.

“Eliminating race-conscious admissions would disproportionately harm applicants of color, including some Asian-Americans,” Harvard alumni said in their filing.

“Applicants’ opportunities to amass credentials that make for a competitive college application are greatly affected by race,” alumni and students wrote. “Given racial bias in standardized testing and endemic racial inequities,” they said the school must continue to consider race.

The Harvard case, which was brought by an anti-affirmative-action group called Students for Fair Admissions, is seen as a test of whether a decades-long effort by conservative politicians and advocates to roll back affirmative action policies will ultimately succeed. The Education and Justice Departments said in July that the administration was abandoning Obama-era policies that asked universities to consider race as a factor in diversifying their campuses and would favor race-blind admissions instead.

Officials from both departments said that the Obama administration had used guidelines to circumvent Congress and the courts to create affirmative action policies that went beyond existing law.

Civil rights leaders and others argue that this stance effectively undermines decades of policy progress that created opportunity for minorities.

The department typically files statements of interest in cases that it believes directly affect the federal government’s interests.

“As a recipient of taxpayer dollars, Harvard has a responsibility to conduct its admissions policy without racial discrimination by using meaningful admissions criteria that met lawful requirements,” Attorney General Jeff Sessions said in a statement.

In briefs filed in support of Harvard at the end of July, students and alumni said that they “condemn” the plaintiffs’ “attempt to manufacture conflict between racial and ethnic groups in order to revive an unrelenting agenda to dismantle efforts to create a racially diverse and inclusive student body through college admissions.”

“It’s alarming that Trump is aligning himself with anti-civil rights activist Edward Blum in this subversive attempt to say that civil rights protections cause discrimination,” said Jeannie Park, the head of the Harvard Asian-American Alumni Alliance and co-founder of the Coalition for a Diverse Harvard, referring to the founder of Students for Fair Admissions. “Trump does not speak for Asian-Americans, just as Blum does not.”

At the heart of the case is whether Harvard’s admissions staff hold Asian-Americans to higher standards than applicants of other racial or ethnic groups, and whether they use subjective measures, like personal scores, to cap the number of Asian students accepted to the school.

“Harvard today engages in the same kind of discrimination and stereotyping that it used to justify quotas on Jewish applicants in the 1920s and 1930s,” Students for Fair Admissions said in a court filing.

Harvard, which admitted less than 5 percent of its applicants this year, said that its own analysis did not find discrimination.

A trial in the case has been scheduled for October.

If it winds its way to the Supreme Court, it could be heard by Judge Brett M. Kavanaugh, Mr. Trump’s nominee for the vacant seat once held by Justice Anthony M. Kennedy. The case could have far-reaching implications for the nation’s colleges and universities that consider race in their admissions processes.

The Justice Department is pursuing its own investigation into Harvard’s admissions policies based on a complaint it received.

The Obama administration used statements of interest in novel ways to further its civil rights agenda, sometimes by making the unusual move of intervening in local cases.

In 2013, the Justice Department intervened in a local case in Burlington, Wash., where poor people contended that their public lawyers were too overworked to adequately do their jobs. The Obama administration also intervened in local legal disputes over legal aid issues in New York, juvenile prisoners in California and transgender students in Michigan.

An earlier version of this article misstated when the Justice Department released a statement of interest supporting students suing Harvard in an affirmative action case. The filing was made Thursday, not Friday.